How to execute an UPDATE only if one row would be affected?

Question

I have a table in SQL Server that has a PK (ID) and another (logical) primary key made by a couple of other columns (although there is no UNIQUE constraint on that)

Answer 1

I'd put the conditional before the update rather than in the where clause.

IF (SELECT COUNT(*) FROM PERSON WHERE NAME = 'XX' AND SURNAME = 'YYY') = 1
UPDATE PERSON SET AGE = 43 WHERE NAME = 'XX' AND SURNAME = 'YYY'